Primary adaptive mirrors for ELTs: a report on preliminary studies

1. Primary adaptive mirrors for ELTs: a report on preliminary studies Presented by A. Riccardi A. Riccardi1, C. Del Vecchio1, P. Salinari1, G. Brusa1,2, O. Lardiere1, P. Mantegazza3, D. Gallieni4, R. Biasi5, R. Spairani6 1INAF-Osservatorio Astrofisico di Arcetri, Firenze, ITALY 2Steward Observatory, Tucson, AZ, USA 3Politecnico di Milano, Milano, ITALY 4ADS Srl, Lecco, ITALY 5Microgate Srl, Bolzano, ITALY 6Solaria Srl, Pavia, ITALY

2. LBT: Gregorian672 actuators MMT: Cassegrain336 actuators 642mm 911mm Current AS technology

3. Current AS technology Cap. sensor armatures (ref.plate) MMT336 ASPHERIC SHELL 642mm diam. 2mm thick 31mm Magnets (12mm diam)

4. Jan 2003 Nov 2002: 1st light at MMT (Mt. Hopkins, AZ, USA) On-sky proven technology See Brusa or Lloyd-Hart, San Diego SPIE

5. Telescope Adaptive Mirrors • MCAO is a real need for ELTs. MCAO requires large number of actuators, up to some 100k for good correction in V-R band distributed in several correctors (typically 2-4). • AS technology allows to use existing telescope optics as correctors without any need of additional relay optics. • Moving from active to adaptive mirrors allows to relax optical tolerances and reduce the glass mass. • The mass reduction and the large control bandwidth of adaptive (wrt active) mirrors allows to relax the rigidity requirements on the ELT structure and control piston with higher bandwidth.

6. Exo-planet imaging Aliasing at Nyquistspatial frequencyat l/(2d) 0.1arcsecexo-earth@10pc Normalized PSF intensity arcsec See poster by O. Lardiere et al.

7. Exo-planet imaging 10cm actuator separation See poster by O. Lardiere et al.

8. L>>30mm L<<30mm Gravitational quilting Shell thickness T ~ L2 1.5mm Turb. correction Ft ~ T3/L2 ~ L4 T Dissipated Power in the coil P ~ (F/e)2 ~ L5 L-7 L Requested electronic/natural damping D ~ BW M ~ L4 Constrains in scaling technology Development target: ~100mm/act projected on sky Current technology: 30mm/act (OK for Owl secondary) L << 30mm requires too thin shells to limit power dissipation Large shell manufacturing, magnet gluing, magnet interaction and shell handling became critical L >> 30mm has a solution to reduce the power dissipation and dampingdemand: load-spreader or slave-actuators. More support points, lower thickness is needed, lower shell thickness (power), lower mass (damping)

9. Adaptive primary concept 1 segment electronicscontrol 100-200 actuators.Less then LBT AdSec.Control electronics forone panel is not an issue

10. No need ofdirectcooling Scaling AS technology to Primary r0=30cm@R (15cm@V), Target SR=0.92@R, Quilting<8 nm WF rmsL=10 cm (100act/m2), Settling time=0.5 ms, stroke=0.5 mm Multi-coil act.+moving coil Load-spread.+ moving coil Backaskog 1999 Shell thickness: Glass weight per act: Act. moving mass: Act. efficiency Power weight support: Power turb. correction: Damping per actuator: (for 0.5ms settling time) 10 mm 244 g 170 g 4 N/W1/2 1 W/act (100 W/m2) 63 mW/act (6 W/m2) 2000 Ns/m 2.8 mm 61 g 41 g 4 N/W1/2 resid. 1% (6 mW/m2) 20 mW/act (2 mW/m2) 500 Ns/m 2.8 mm 61 g 21 g 3 N/W1/2 Resid. 1% 7 mW/m2 66 mW/act (6 mW/m2) 400 Ns/m Weight can be supported by a spring. The springs gives a piston term when thegravity direction changes. That can be compensated by pistoning the whole panelusing its hexapod

11. 50 50 Actuators Electromagnet actuator Commercialmoving coil actuator(Bei Kimco) Moving magnet actuator

12. From J. Burge et al., SPIE Proc., 4451, 2001 From MNDS Status review, 2001 Carbon fiber mirrors using replica? Shell manufacturing • 2mm-thick 2m-diameter class shell feasibility has been proved • Studies for mass production are needed for glass shell

13. Edge actuators canbe reduced to: 96 edge actuators Tot: 265 actuator/panel Gravitational quilting nm Thickness = 2.8mm VV size = 1.6m Borosilcate 169 triple acts 144 edge actuators Tot: 313 actuators/panel Rms focus-removed: 9nm (wavefront)

14. 1.6m v-v hex segments 100m Quilting vs. piston error

15. Quilting vs. piston error

16. Influence functions The edge is nicely controlled No print-through on the three-tips of act.

17. Natural damping Gderiv = 0 Gderiv = 0 LP comm Settling time: 0.7ms Settling time: 2.7ms 2.4 times faster Settling time: 1.7ms Damping for bandwidth Digital damping Gap=67mm Gprop = 0.1N/mm Gderiv = 25Ns/m Dnatural = 17Ns/m

18. Conclusions • Exo-Earth detection requires high density corrector (10cm pitch) • AdSec technology can be used directly on segments of primary mirror without need of actuator cooling. • Adaptive primary bandwidth allows to remove very effectively differential piston error of segments. • Glass shells are already produced, but mass production studies are still needed. Carbon fiber shells with replica is an attractive alternative for mass production. • Gravitational quilting error is not an issue. • Control electronics per panel is not an issue. • Electronic damping has been tested on a small prototype

19. The End

20. Outline

21. On-sky proven technology

22. Gravitational quilting T L Constrains in scaling technology Development target: ~100mm/act projected on sky Current technology: 30mm/act (OK for Owl secondary) Large L Small L Shell thickness T ~ L2 Const Turbulence correction Ft ~ T3/L2 ~ L4 L-7 Weight support Fw ~ T L2 ~ L4 Actuator efficiency e=F/P1/2 ~ V1/2 ~ L3/2 Dissipated Power in the coil P ~ (F/e)2 ~ L5 Requested electronic/natural damping D ~ BW M ~ L4

23. Act. sep. [mm] Act. sep. [mm] Act. sep. [mm] Constrains in scaling technology l-7 l5 l4 l2 Shell Thickness [mm] Damping/act [Ns/m] Power/act [W] 1.4mm 36mm 36mm L << 36mm requires too thin shells to limit power dissipation Large shell manufacturing, magnet gluing and handling became critical L >> 36mm has a solution to reduce the power dissipation and dampingdemand: load-spreader or slave-actuators. More support points, lower thickness is needed, lower shell thickness (power), lower mass (damping)

24. Gravitational quilting. Horizon pointing 2.4 nm rms wavefront

25. Panel section